In recent years, networks such as wireless sensor networks have emerged as an ideal solution to a number of applications such as monitoring, target tracking, biological/chemical attack detection, and commercial and residential automation. Wireless sensor networks share many common traits with existing wireless ad-hoc network concepts, but there are also a few differences and specific challenges. One of the challenges is that energy is typically scarce in a wireless sensor network and it is difficult and sometimes impossible to replenish the source or sources of that energy. Therefore, protocols that save power need to be developed to conserve energy in wireless sensor networks. For example, there is considerable interest currently in designing and developing a highly energy efficient Medium Access Control (MAC) protocol for wireless sensor networks.
A MAC protocol typically encapsulates payload data with a header before the data and a Cyclic Redundancy Check (CRC) after the data. The entire frame is preceded by a small idle period and a preamble. The idle period is provided to allow a small time interval for the receiver electronics in the nodes of a network to settle after processing previous frames. The preamble is provided to allow the node receiver to synchronize its data clock to the transmit data clock. The header consists of a destination address, a source address, and a type field.
The major sources for the depletion of the energy source of a node are overhearing, idle listening, and packet collisions. In overhearing, a node wastes energy in receiving packets that are not addressed to it. Idle listening is the state in which a node does not know when it will be the receiver of a message from one of its neighbor nodes. So, the node must continuously maintain itself in the receive mode, which wastes energy. In case of packet collisions, the node has to retransmit a packet that is involved in collisions with one or more other packets, which again wastes energy.
Classes of contention-based MAC protocols, like carrier sense multiple access (CSMA) protocols, are not suitable for wireless sensor networks because they waste energy. A reservation based MAC protocol such as code division multiple access (CDMA) is not preferred in a sensor network because it requires special hardware and consumes substantial energy during encoding and decoding of information. Frequency division multiple access (FDMA) can be used in wireless sensor networks but it is costly compared to TDMA systems.
Many TDMA slot assignment algorithms have been proposed for wireless sensor networks. For example, a five phase reservation protocol (FPRP) has been previously proposed to employ a contention-based mechanism in which nodes compete with each other to acquire TDMA slots. With the help of a five-phase algorithm, a final reservation schedule for the whole network is generated.
Almost all TDMA algorithms that have been proposed for wireless sensor networks generate a communication schedule for the entire network. For some applications, it is not necessary to assign slots to all nodes in the network at the same time. For example, in a target-tracking application, only the nodes in the vicinity of a target should participate in data gathering, and these nodes require a communication channel to transmit the gathered data. However, all of the other nodes are idle and are not part of the data gathering and communication process. Therefore, it is not necessary to assign time slots to these other nodes, which results in a better utilization of the channel. Not assigning time slots to certain nodes requires a change in the usual TDMA approach. Accordingly, either a reactive TDMA algorithm or a modified proactive TDMA algorithm that assigns time slots to nodes on demand is suitable for target tracking kinds of applications.
The present invention is directed to a neighbor base assignment of time slots.